In conventional processes for melting metals, particularly iron or copper, Cupola furnaces are used with coke as the fuel. Processes are also known in which, a part from coke, fuels such as fuel gas, mineral oil or coal waste products are used. However, these processes suffer from the important disadvantage that the molten metal is contaminated by the use of coke as fuel, mainly by sulphur and phosphorus, or alternatively coke with the necessary quality is not available.
Therefore processes for melting metals in shaft furnaces are known in which coke not used as the fuel and the metal is instead melted with the aid of fuel gases such as natural gas, city gas, propane and butane, or mineral oils. These processes are advantageously characterized in that no contaminants or impurities are introduced by the fuel into the molten metal, because the contact time between the gaseous combustion products and the metal charge is relatively short.
Such a process is known from DE-AS No. 22 04 042, which describes a process for melting iron in a vertical shaft furnacne without the use of coke and using a fluid fuel/air mixture. This mixture is burned in burners outside the furnace and which are located close to the base of the furnace. Scrap iron and pig iron are fed into the upper end of the furnace and molten, superheated or overheated metal passes through a bed of loose refractory elements arranged within the shaft. Said elements are heated by the upwardly travelling combustion products of the burners before the combustion products melt the oncoming metal. The molten metal is tapped at the bottom of the furnace. The refractory elements forming the bed preferably comprise lime-bonded galena graphite. During the operation of the shaft furnaces, they are consumed in a quantity of 2% or less with respect to the weight of the molten iron, so that the furnace can only be operated for relatively short periods before the bed has to be completely replaced. The bed can be kept at its working depth, in that the bed material is fed into the introduced charge materials in a quantity of approximately 1 to 2% of the weight of the metal charge.
The article "Der kokslose giessereischachtofen" by W. Sachs, Giesserei 66, No. 12, 11.6.1979, pp 415 to 417 also describes a process for melting metal in a shaft furnace, in which use is made of a gaseous or liquid fuel. Balls of ceramic material on a water-cooled grate or grid are used as the superheating bed. Here again ceramic balls must be added to each charge to maintain the height of the superheating bed.
The article "Das Umschmelzen von Kupferschrott im Flaven-ofen" by Dr. H. P. Goossens in the journal ERZMETALL, vol 24, No. 3, March 1971, pp 105 to 107 also describes a shaft furnace operating without coke as the fuel material. The furance bed is held by a grid or grate made from water-cooled steel pipes, which on the one hand prevents the solid charge from dropping from the shaft furnace into the underlying hearth and on the other hand in counter flow ensures the through-flow of the hot combustion gases. The grid carries an approximately 20 cm high layer of highly refractory materials, e.g. corundum lumps or coke with limestone.
The three aforementioned known shaft furnaces, which do not use coke as the fuel, consequently have the common feature that in operation the bed material is necessary for superheating the melting charge and is subject to wear during the melting process. The need for a superheating bed results from the fact that the oxygen provided for burning the fuel has to be supplied substoichiometrically to ensure that there is no oxidation of the molten metal through unburnt oxygen. Therefore the combustion of the fuel is not complete, so that the flame temperature which can be reached is relatively low. Therefore for melting the metal the contact time between the latter and the material supplying heat thereto must be made as long as possible and the superheating bed fulfills this function. The ceramic materials of this bed are subject to slagging and coke is consumed through the gas component CO.sub.2 and H.sub.2 O via the Boudouard or water gas reactions. It is therefore necessary to constantly supply to the metal charges fed into the furnaces a new bed material quantity which compensates for wear.
When using ceramic or coke as the bed material there are relatively large amounts of slag, which impair the economics of the furnace, because a considerable energy proportion is required for heating, forming and superheating the slag. As a result of the relatively high weight of the bed materials, large and expensive amounts thereof are required. When using ceramic bed material it is subject to premature softening, because it is not resistant to attacks by slag.
The earlier-dated application No. P 34 37 911.8 describes a process for melting metal in a shaft furnace using a liquid or gaseous fuel, where the lower section of the furnace contains a separating and superheating bed for the metal, which is heated by the combustion products passed from below the bed upwards through the shaft furnace, so that the metal is melted and is collected in this state below the bed. The combustion air supplied to the burner is previously heated, said combustion taking place recuperatively through the waste gases of the shaft furnace. This raises the flame temperature of the burner and improves the economic operating characteristics of the shaft furnace. However, even this known process still requires a superheating bed and the flame temperature cannot be controlled by means of the degree of heating of the combustion air.
The problem of the present invention is therefore to improve the process disclosed in patent application No. P 34 37 911.8 in such a way that the desired temperature for the molten metal can be adjusted in a simple manner and can be achieved even without the use of a superheating bed.